Breeding Strategies For Development of Climate Resilient Varieties In Rice

Heat tolerance is defined as the ability of the plant to grow and produce economic yield under high temperature (Wahid et al., 2007). Tolerance to heat comprises of escape or avoidance mechanisms like timing of panicle emergence, spikelet opening during stress period, anther dehiscence. Heat shock proteins are considered to be the stabilizing factors conferring tolerance to heat thereby protection of structural proteins, enzymes and membranes from heat damage is crucial in temperature tolerance. (Maestri et al., 2002).

Plant architecture is an important trait for tolerance to temperature stress. As an example in some genotypes the panicle is surrounded by many leaves the plant will be able to withstand high temperature stress due to increased transpirational cooling and prevention of evaporation from anther due to shading of leaves. The early morning flowering of rice plant is a useful phenomenon imparting heat tolerance to rice plant. These traits can easily be used in breeding programmes as it inherits in simple manner (Yoshida, 1981). Genotypes of Oryza glaberrima flower much earlier in the day with more than 90% of the spikelets reaching anthesis by 09.00 h. (Prasad et al., 2006). This trait can be used for introgression in O. sativa genotypes. It has been reported that cultivars with large anthers are tolerant to high temperatures at the flowering stage. (Matsui and Omasa, 2002).

For high temperature tolerance, traits such as spikelet fertility can be used as a screening tool during the reproductive stage. Selection of heat tolerance should be done for those materials which can tolerate temperatures higher than 38oC. Cultivars such as N22 has already been identified as high-temperature tolerant so these material can be used in breeding programmes as donors. Genetic modification of the male reproductive organs should be emphasized as it is more sensitive to high temperature. Candidates genes can be identified using QTL mapping, by studying the association of the phenotype and its associated markers. Identification and breeding of heat tolerant germplasm should be carried out for exploiting variation both in genotypic and morphological characters.

Most of the rice is currently grown in those areas where the current temperatures are close to optimum. By the end of 21st century the rice yields have been estimated to be reduced by 41% (Ceccarelli et al., 2010). It has been evidenced that increase in night temperature has been the main cause of increase in global mean temperature resulting in decrease yields (Peng et al., 2004 and Sheehy et al., 2005).

Effect of high temperature on rice plant

The optimum temperature for the normal rice development ranges from 27 to 32oC (Yin et al., 1996). Flowering and the booting stage in rice is considered to be most susceptible to temperature. Temperature higher than the optimum induced floret sterility and decreased rice yield (Nakagawa et al., 2003). High temperature causes floret sterility and decreased ability of pollen grains to swell, resulting in poor pollen dehiscence. Temperature increase of 1oC shortened the number of days from sowing to heading in some genotypes. The symptoms of heat stress in rice has been shown in Table 1.

Mechanisms of heat tolerance and Breeding strategies

Heat tolerance is defined as the ability of the plant to grow and produce economic yield under high temperature (Wahid et al., 2007). Tolerance to heat comprises of escape or avoidance mechanisms like timing of panicle emergence, spikelet opening during stress period, anther dehiscence. Heat shock proteins are considered to be the stabilizing factors conferring tolerance to heat thereby protection of structural proteins, enzymes and membranes from heat damage is crucial in temperature tolerance. (Maestri et al., 2002).

Plant architecture is an important trait for tolerance to temperature stress. As an example in some genotypes the panicle is surrounded by many leaves the plant will be able to withstand high temperature stress due to increased transpirational cooling and prevention of evaporation from anther due to shading of leaves. The early morning flowering of rice plant is a useful phenomenon imparting heat tolerance to rice plant. These traits can easily be used in breeding programmes as it inherits in simple manner (Yoshida, 1981). Genotypes of Oryza glaberrima flower much earlier in the day with more than 90% of the spikelets reaching anthesis by 09.00 h. (Prasad et al., 2006). This trait can be used for introgression in O. sativa genotypes. It has been reported that cultivars with large anthers are tolerant to high temperatures at the flowering stage. (Matsui and Omasa, 2002).

For high temperature tolerance, traits such as spikelet fertility can be used as a screening tool during the reproductive stage. Selection of heat tolerance should be done for those materials which can tolerate temperatures higher than 38oC. Cultivars such as N22 has already been identified as high-temperature tolerant so these material can be used in breeding programmes as donors. Genetic modification of the male reproductive organs should be emphasized as it is more sensitive to high temperature. Candidates genes can be identified using QTL mapping, by studying the association of the phenotype and its associated markers. Identification and breeding of heat tolerant germplasm should be carried out for exploiting variation both in genotypic and morphological characters.

Climate change has been a hot topic nowadays and its impact on agriculture and related fields makes the scientific community to work towards innovating new technologies which proves resilient during fluctuations in climate. In a report by IPCC (2001) which states that in the past century the temperature have increased by more than 0.6oC. It is very surprising to know that most of the warming has occurred since the 1970s and also the warmest years has occurred in the past decade. Further, looking at the last 1000 years, the most warmest years have occurred in the last 60 years and this has caused rise in the occurrence of floods and drought (Wassmann and Dobermann, 2007).

As a C3 plant the rise in Co2 concentration will have beneficial effect on rice plant but the overall effect in the tropical areas will be negative. Erratic rainfall and extreme weather events will increase frequencies of both drought and floods. Higher temperature affect the rice crop particularly during the pollination stage which results in more sterile grains and thus less yield. Increase in sea level will cause inundation of more coastal areas and increase in salinity problem of the coastal areas. Change in climate will have effect on insect pest and diseases. Some of the pathogen and insect pest may proliferate and cause epidemics in rice. Drought and floods will cause change in water use efficiency and nutrient use efficiency of the crop and also the nutrient uptake of the rice due to change in the soil microclimate. Rice crop suffers from a number of stresses which hamper the rice production directly or indirectly. Stresses like drought, cold, heat, disease/insect and flooding affects the rice crop economically. It is estimated that the frequency of these stress environment will increase in the near future.

Plant breeding technologies often combine traditional knowledge with cutting edge biotechnological techniques are already making real impact in meting the challenge of climate change. Apart from crop management strategies for climate change Plant Breeding plays a major role in combating this change by evolving such genotypes which can withstand in stress environments. Breeding climate resilient varieties is a comprehensive approach for mitigating the effects of climate change on rice.

The integration of conventional breeding techniques with modern biotechnological approaches which covers the genomics, proteomics and phenomics aspects of the crops makes the breeding process more efficient and evolving the new rice varieties in much shorter time. Genetic resources are a store house for alleles that provide resilience to the crop under various stresses. The traditional cultivars are valuable germplasm which can be used in breeding programmes. MAS for climate resilient traits in rice have proved to be effective in varietal development. QTL mapping for genes conferring resistance to various stresses in rice is quite effective methodology for mapping genes and its introgression in elite varieties. Here we discuss various stresses in rice due to climate change and the breeding strategies for mitigating the stress and development of varieties which will be the future weapon to cope with climate change.